Project Summary Despite that the identification of the Zika virus occurred decades ago, little is known about this virus and its interactions with the human immune system. With the confirmation of serious complications from infection with this virus, scientists are now scrambling to decipher the virological, immunological, clinical, and social aspects of Zika. The end goal, understandably, is to ultimately use this knowledge in order to devise tools for prevention and treatment of Zika infections. There are many strategies that can be pursued in this quest, including small-molecule drugs or antibody-based therapeutics. Vaccines, however, remain a top priority target as preventive measures due to factors such as lower costs and lower barriers to adherence, among others. A variety of Zika vaccine platforms are already being developed, including DNA-based and live attenuated candidates. Recombinant subunit vaccines are of special interest due to the potentially easier production and lower possibility of adverse effects. Here, we propose to develop vaccine candidates based on the Zika E protein, a primary target of antibody responses. This project will perform immunogen development for Zika virus starting from concept, through design, to experimental validation (both in vitro and in vivo). The output from this project will be vaccine candidates that can elicit potent Zika-neutralizing antibody responses in animal models. Because of the recently confirmed antibody-dependent enhancement between Zika and dengue, we propose that an effective Zika vaccine will require the elicitation of potent broadly neutralizing antibody responses against both Zika and the dengue viruses. To achieve this, we propose to apply structure-based design strategies using several different approaches, including (a) immunogen design targeting the incorporation of E protein antigens from both Zika and the different dengue viruses, and (b) immunogen design for focusing the antibody response toward conserved epitopes that are known targets of Zika/dengue broadly neutralizing antibodies. Lead immunogen candidates will be evaluated in animal models for their ability to elicit broadly neutralizing antibody responses, without leading to antibody-dependent enhancement of Zika or dengue infection. Taken together, this proposal will aim at building a foundation for the design of effective vaccines against the Zika virus, with a special consideration of the potential impact of prior or future infections with dengue viruses. Moreover, the immunogens designed here will be of general value as vaccine candidates potentially capable of protecting simultaneously against several different viruses. Overall, the immunogen design technology that we are proposing to develop for Zika should be applicable to other flaviviruses, and potentially other viruses, and therefore has the potential for a far-reaching impact on public health.